1. Field of the Invention
The present invention relates generally to cutting tools and more particularly, but not by way of limitation, to cutting tools used in roadway resurfacing machines.
2. Description of the Prior Art
In the course of time, the effects of traffic and weather on paved roadways result in the development of cracks, pot holes and the like so that roadways are continually maintained and repaired. For a time after a roadway is constructed, this maintenance may consist of nothing more than simply patching defects as they appear; for example, pot holes may be filled with asphalt. Eventually, however, such minor repair will be insufficient to restore a roadway to reasonable condition. In the past, it has been common to deposit a new layer of paving material on the roadway when such a condition is reached. Numerous roadways in the United States, both highways and city streets, have been relayered a number of times.
While relayering is an effective means of extending the lifetime of a paved roadway, the process cannot be continued indefinitely. Eventually, the roadway will be built up to a level which makes relayering impractical. Moreover, if the relayering is carried out on a roadway having pot holes, the new surface will not distribute a load thereon as evenly as the surface of a newly constructed roadway so that deterioration of the roadway occurs at an accelerated rate. To counter these problems, resurfacing machines have been developed to remove layers of paving materials rather than to deposit new layers. These machines provide a number of advantages in the art of roadway maintenance. For example, the roadway may be usable in the condition in which it is left by the resurfacing machine so that resurfacing results in a direct extension of the lifetime of the roadway. Such a result is not uncommon where many layers of asphalt have been deposited on a roadway over a number of years.
Even where the roadway will not be usable in the condition in which it is left by a resurfacing machine, it is nevertheless advantageous to resurface the roadway as an intermediate step to relayering. By removing internal defects, a better load distributing capacity may be achieved to extend the lifetime of the newly deposited surface and, since the resurfacing operation leaves an even surface for the new layer, the layer may be thinner than would be required were the roadway not resurfaced prior to relayering. The resulting savings in both money and resources justify the cost of resurfacing.
The manner in which roadways have been constructed in the past has presented a problem in the use of the resurfacing machines so that the potential savings offered by these machines may not be fully achieved. Roadways are commonly constructed of either asphalt or concrete. Experience has shown that the requirements of asphalt and concrete roadway resurfacing differ.
In general, a roadway resurfacing machine incorporates a drum having a flight wrapped spirally thereabout. A number of cutting heads are attached to the flight and the drum is rotated as the machine is moved along the roadway so that cutters, forming a part of the cutting heads, are driven into and cut away portions of the roadway. It will be recognized that the forces existing between the roadway and the cutters will be quite large. Since these forces are exerted on a leading edge of the cutter, the cutter tends to twist in its holder so that a rotatably mounted cutter undergoes auto-rotation as it cuts away the surface of a roadway.
It has been found that auto-rotation of cutters having conically shaped bits is advantageous when the roadway being resurfaced is made of concrete. The texture of the resulting surface has desirable characteristics both in terms of use as is and in terms of relayering. Moreover, the bits are not subject to excessive wear so that the cost of resurfacing is maintained at a reasonable value.
A different situation obtains when an asphalt roadway is resurfaced. The auto-rotation of the cutters and the shape of the bits result in excessive wear of portions of the cutters to which the bits are attached. For asphalt roadways, non-rotating spade type bits; that is, rectangular bits having a cutting edge formed along one side, have been found to be advantageous. The wear rate of such cutters is well within acceptable limits and the resulting surface is suitably textured.